System and method for treating skin

ABSTRACT

System and method for treating a skin target. A temperature effector creates a temperature difference between the target and the skin tissue surrounding the target such that the target is at a higher temperature than the surrounding the. One or more RP electrodes are attached to the skin and RE energy is applied.

FIELD OF THE INVENTION

[0001] This invention relates to methods and systems for treating skin.

BACKGROUND OF THE INVENTION

[0002] The term “target” is used herein to denote a skin defect such asa vascular lesion, pigmented lesion, acne, unwanted hair or wrinkle.Selective thermal treatment of skin is commonly used in aestheticmedicine to remove skin targets. In order to be destroyed, the targetmust be raised to a temperature of about 70° C. without raising thetemperature of the surrounding epidermis or dermis to damaging levels.The most popular method of thermal skin treatment is selectivephoto-thermolysis in which light energy produced by a laser or flashlamp is selectively absorbed by a pigmented portion of the target.However, with this method it is often not possible to heat the entiretarget to a temperature necessary for destroying it without heating thesurrounding skin to damaging levels. The main problem is that theoptical contrast between the target and the surrounding skin tissue isnot high enough to obtain a significant difference in temperaturebetween the target and the surrounding skin tissue.

[0003] U.S. Pat. No. 5,755,753 disclose use of the radio-frequency (RF)range of electro-magnetic energy for skin tightening, where RF energy isapplied to a pre-cooled skin surface. U.S. Pat. No. 5,846,252 disclosestreating hairs to reduce their electrical resistance and then applyingRF current.

SUMMARY OF THE INVENTION

[0004] The present invention is based upon the finding that selectiveheating of a skin target by RF energy is enhanced if prior to theapplication of the RF energy the skin is treated to make the temperatureof the target (T_(t)) higher than the temperature of the surroundingskin tissue (T_(s)). The initial temperature gradient (T_(t)−T_(s)>0)between tie target and surrounding tissue may be achieved either bypreheating the target or pre-cooling the surrounding tissue.

[0005] The invention thus provides a system for treating a skin targetcomprising:

[0006] (a) one or more RF electrodes configured to be attached to theskin, so as to apply an RF current to the skin;

[0007] (b) a temperature effector configured to create a temperaturegradient between the target and skin surrounding the target such thatthe target is at a higher temperature than the surrounding skin.

[0008] The invention still further provides a method for treating a skintarget comprising:

[0009] a) creating a temperature gradient between the target and skinsurrounding the target such that he target is at a higher temperaturethan the surrounding skin; and

[0010] b) applying RF energy to the skin.

[0011] The system and method of the invention may be used for such skintargets as a vascular lesion, pigmented lesion, hair follicle, wrinkleand acne.

[0012] While not wishing to be bound by a particular theory, it isbelieved that selective thermolysis of a target by RF energy is enhancedwhen T_(t)−T_(s)>0 due to an increase in the electrical conductivity inthe RF range of tissues when thee tissue temperature is increased[Frances A. Duck, Physical Properties of Tissue, a ComprehensiveReference Book, Academic Press, 1990, p.173]. Accordingly, thedependence of the conductivity a of a tissue on temperature T is givenby:

σ=σ₀(1+α(T−T ₀))  (1)

[0013] where σ₀ is the conductivity at the reference temperature T₀ andα is a constant known as the temperature coefficient.

[0014] Heat generation by RF current can be estimated by Joule's Law:

H=σE ²  (2)

[0015] and the change in temperature in the tissue is obtained using theheat conductivity equation: $\begin{matrix}{{c\quad \rho \frac{\partial T}{\partial t}} = H} & (3)\end{matrix}$

[0016] where c is the heat capacity of the tissue, ρ is the mass densityand E is the intensity of the electric field.

[0017] Inserting Equations 1 and 2 into (3), $\begin{matrix}{{c\quad \rho \frac{\partial T}{\partial t}} = {\sigma_{o}\left( {1 + {{\alpha \left( {T - T_{o}} \right)}E^{2}}} \right.}} & (4) \\{{{Setting}\quad A} = \frac{{\alpha\sigma}_{o}E^{2}}{c\quad \rho}} & (5)\end{matrix}$

[0018] and integrating Equation 4, the result is $\begin{matrix}{T^{\prime} = {T_{o} + \frac{^{At} - 1}{\alpha} + {\left( {T_{i} - T_{o}} \right)^{At}}}} & (6)\end{matrix}$

[0019] where T_(i) is the initial temperature of the tissue before theapplication of RF energy, t is the duration of the application of RFenergy, and T′ is the formal temperate of tie tissue at the end of theapplication of RF energy.

[0020] If the initial temperatures of the target and sounding skintissue are T_(t) and T_(s) respectively (T_(t)T_(s)>0), then Equation 6becomes for the target: $\begin{matrix}{T_{t}^{\prime} = {T_{o} + \frac{^{{At} - 1}}{\alpha} + {\left( {T_{ij} - T_{to}} \right)A\quad ^{it}}}} & (7)\end{matrix}$

[0021] and for the surrounding skin, $\begin{matrix}{T_{s}^{\prime} = {T_{0}^{\prime} + \frac{^{At} - 1}{\alpha} + {\left( {T_{st} - T_{o}} \right)^{At}}}} & (8)\end{matrix}$

[0022] subtracting Equation (8) from Equation (7) yields

T′ _(t) −T′ _(s)=(T _(ti) −T _(si))e ^(At)  (9)

[0023] where T_(ti)−T_(si) is the initial temperate gradient between thetarget and the surrounding skin, and T′_(t)−T′_(s) is the finaltemperature gradient. Equation (9) shows that as the RF current isapplied, the temperature gradient increases exponentially. Therefore, bycreating an initial relatively small temperature gradientT_(ti)−T_(si)>0, and applying RF energy, a larger temperature gradientis obtained. This allows heating of the target to a sufficiently hightemperature to destroy the target without heating the surrounding skintissues to damaging levels.

[0024] Assuming a typical RF fluence (F) in the skin of 20 J/cm², α=0.03(C°)⁻¹ and a heat capacitance cρ=3.6 J/cm³°K the factor e^(At) inEquation (9) is$^{At} = {{e\frac{{\alpha\sigma}_{o}E^{2}}{c\quad \rho}} = {{e\frac{\alpha \quad {Ht}}{c\quad \rho}} = {^{0.83} = 2.3}}}$

[0025] Thus, the temperature gradient increases by a factor of about 2.3during the application of the RF energy.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] In order to understand the invention and to see how it may becarried out in practice, a preferred embodiment will now be described,by way of non-limiting example only, with reference to the accompanyingdrawings, in which:

[0027]FIG. 1 shows a system for heating a skin target and applying RF toan individual in accordance with one embodiment of the invention;

[0028]FIG. 2 shows a method for treating skin using the system of FIG.1;

[0029]FIG. 3 shows an applicator with two electrodes, and a light sourceused in tie system of FIG. 1.

[0030]FIG. 4 shows a system for cooling skin surrounding a target andapplying RF energy to an individual in accordance with anotherembodiment of the invention;

[0031]FIG. 5 shows an applicator with two electrodes, and a coolingsystem used in the system of FIG. 3; and

[0032]FIG. 6 shows a method for treating skin using the system of FIGS.4 and 5.

DETAILED DESCRIPTION OF THE INVENTION

[0033] Referring to FIGS. 1 and 2, a system for creating a tempergradient between a skin target and the surrounding skin, in accordancewith the invention is shown. An applicator 703, to be described indetail below, contains a pair of RF electrodes 401 and 402 and a lightsource 403. The applicator 703 is adapted to be applied to the skin ofan individual 705 in the region of a target. The applicator 703 isconnected to a control unit 701 via a cable 702. The control unit 701includes a power source 708. The power source 708 is connected to an RFgenerator 715 that is connected to the RF electrodes in the applicator703 via wires in the cable 702. The power source 708 is also connectedto a light source 403 in the applicator 703 via wires in the cable 702.The control unit 701 has an input device such as a keypad 710 thatallows an operator to input selected values of parameters of thetreatment, such as the frequency, pulse duration and intensity of the RFenergy or the wavelength and intensity of the optical energy. Thecontrol unit 701 optionally contains a processor 709 for monitoring andcontrolling various functions of the device. For example, the processor709 may monitor the electrical impedance between the electrodes in theapplicator 703, and determine the temperature distribution in thevicinity of the target. The processor 709 may also determine theparameters of the treatment based upon the impedance measurements.

[0034]FIG. 2 shows the applicator 703 in detail. The applicator containsa pair of electrodes 401 and 402 that apply RF energy to the skin. Alight source 403 produces a light spectrum that is delivered to the skinsurface by light guide 404.

[0035] In accordance with the method of the invention, the system shownin FIG. 1 is used to first apply optical energy to a target having adiameter for example of 2 mm. The optical energy may have an intensityfrom about 5 to about 100 Joules/cm² and may be applied from about 1 to200 msec.

[0036] The parameters of RF energy may have the following exemplaryvalues:

[0037] Frequency of the RF energy: from about 300 kHz to about 100 MHz.

[0038] Output power of the RF energy from about 5 to about 200 W.

[0039] Duration of the irradiation: from about 1 to about 500 msec.

[0040] Pulse repetition rate: from about 0.1 to about 10 pulses persecond.

[0041]FIG. 3 shows a flow chart for a method of treating skin using thesystem shown in FIGS. 1 and 2. In step 300, the applicator 703 isapplied to the skin of an individual in the region of a target in theskin. In step 310 the light source 403 is activated so that the targetis irradiated with optical energy from the light source 403 conductedthrough the optic fiber 404 to the target. In step 320 the irradiationwith optical energy is terminated. RF energy is then applied to the skin(step 330). Finally, in step 340, the application of RF energy isterminated.

[0042] Referring now to FIGS. 4 and 5, a system for creating atemperature gradient between a skin target and the surrounding ski, inaccordance with another embodiment of the invention is shown. Anapplicator 803, to be described in detail below, contains a pair of RFelectrodes 401 and 402. The applicator 803 is adapted to be applied tothe skin of an individual 805 in the region of a target. The controlunit 801 includes a power source 808. The power source 808 is connectedto an RF generator 815 that is connected to the RF electrodes in theapplicator 803 via wires in the cable 802. The control unit 801 controlsa refrigeration unit 812 that cools a fluid such as ethanol or water forcooling the applicator 803. The cooled fluid flows 10 from therefrigeration unit 812 to the applicator via a first tube in the cable802, and flows from the applicator 803 back to the refrigeration unitvia a second tube in the cable 802. The control unit 801 has an inputdice such as a keypad 810 that allows an operator to input selectedvalues of parameters of the treatment, such as the frequency, pulseduration and intensity of the RF energy or the temperature of thecoating fluid. The control unit 801 optionally contains a processor 809for monitoring and controlling various functions of the device. Forexample, the processor 809 may monitor the electrical impedance betweenthe electrodes in the applicator 803, and determine the temperaturedistribution in the vicinity of the target. The processor 809 may alsodetermine the parameters of the treatment based upon the impedancemeasurements.

[0043]FIG. 5 shows the applicator 803 in, detail. The applicator containa pair of electrodes 401 and 402 that apply RF energy to the skin. Thehousing and electrodes are cooled by fluid cooled by the refrigerationunit 812 that flows in a tube 408 between inlet 405 and outlet 406. Theinlet 405 and the outlet 406 are connected to the refrigeration unit 812via the fist and second tubes in the cable 802.

[0044]FIG. 6 shows a flow chart for a method of treating skin using thesystem shown in FIGS. 4 and 5. In step 600, the applicator 703 isapplied to the skin of an individual in the region of a target in theskin. In step 610 cooling system 812 is activated so that the skinsurrounding the target is cooled to a temperature below that of thetarget, RF energy is then applied to the skin (step 620). Finally, instep 630, the application of RF energy is terminated.

1. A system for treating a skin target comprising: (a) one or more RFelectrodes configured to be attached to the skin, so as to apply an RFcurrent to the skin; (b) a temperature effector configured to create atemperature difference between the target and skin surrounding thetarget such that the target is at a higher temperature than thesurrounding tissue.
 2. The system according to claim 1 wherein thetemperature effector heats the target.
 3. The system according to claim2 wherein the temperature effector comprises a light source configuredto apply optical energy to the target.
 4. The system according to claim1 wherein the temperature effector cools the surrounding tissue.
 5. Thesystem according to claim 4 wherein the temperature effector comprisesan irrigation unit cooling a fluid and tubes for allowing the cooledfluid to flow near the surrounding skin.
 6. A method for treating a skintarget comprising: (a) creating a temperature gradient between thetarget and skin surrounding the target such that the target is at ahigher temperature than the surrounding skin; and (b) applying RF energyto the skin.
 7. The method according to claim 6 wherein the temperaturegradient is created by heating the target.
 8. The method according toclaim 7 wherein the target is heated by applying optical energy to thetarget.
 9. The method according to claim 5 wherein the temperaturegradient is created by cooling the skin surrounding the target.
 10. Themet-hod according to claim 9 wherein the surrounding skin is cooled bycontacting the skin with a pre-cooled fluid.
 11. The method according toclaim 6 wherein the target is selected from the group comprising avascular lesion, pigmented lesion, hair follicle, wrinkle and acne.